Hexagon Cake Question

I've searched all over here and the internet, so forgive me if this is a topic that has been covered a lot. I simply cannot find an answer, and I've never worked with hex cakes before.

The only servings I can find for hexagon shaped cakes are 6", 9", 12" and 15". Does anyone have serving sizes for bigger hex tiers? A bride sent me a picture of a cake that she wants, and it is 5 tiers, with the three bottom tiers hex and the two top tiers round. The problem is that she expects 500 people at her wedding. Not 500 invited, 500 expected. Her dad is the pastor at their church (of 900 members) and they've all been invited, plus 150 family/friends invited outside of the church. The only dessert will be this cake and a smaller groom's cake.

So, the way I'm figuring with the bottom hex tiers being 15", 12" and 9", then the top round tiers being 7" and 5", that only gives me around 170 servings. I know I don't necessarily need a hex pan, because I can always cut the shape out of a round or square layer if I needed to. But how big am I going to have to make this sucker to get her close to 300 servings? I say 300 because I told her to split the servings with the groom's cake.

Ok, thanks to the magic of Photoshop I answered my own question. Thankfully a 7" round will sit on top of a 9" hex

Going through some quick numbers 178 for the cake, plus three 11"x15" sheets will yield appx. 410 servings (not counting top tier for ann.), and I think that will work for her. The other servings can be made up in the groom's cake.